I am thinking about the exercise:
Exercise 5. Give an example of a random sequence ($M_n$) such that $E[ M_{n+1} | M_n ] = M_n$ for all $n\ge0$, but which is not a martingale w.r.t. the filtration $F_n = \sigma(M_0, \dots , M_n)$.
From Exercise 5 of Link : http://www.math.bme.hu/~gabor/oktatas/SztoM/StModHW_2014.pdf